1. Field of the Invention
The present invention relates to a discharge lamp lighting apparatus to light a discharge lamp and a lighting apparatus using this discharge lamp lighting apparatus.
2. Description of the Related Art
Fluorescent lamps, metal halide lamps, mercury lamps and high pressure sodium lamps, etc. have been merchandised so far for use as discharge lamps. As conventional discharge lamp lighting apparatus to operate these discharge lamps, there are such apparatus developed that a high frequency oscillation circuit generates high frequency voltage by turning DC voltage from a rectifying means ON/OFF using a switching device in the ON time adjustable state and supplies this high frequency voltage to discharge lamps.
Further, discharge lamps in a small tube diameter of about 15 mm which was so far 30 mm are developed recently. On the other hand, if the tube diameter of a discharge lamp is small, its starting voltage will tend to become high. Therefore, when a discharge lamp in a small tube diameter is an applicable load, it is necessary to set the output voltage of an inverter at a high level.
There are some discharge lamp lighting apparatus which have capacitors connected in parallel to the discharge lamps for preheating filaments. In case of a discharge lamp lighting apparatus with capacitors connected in parallel, preheating current becomes almost constant-current if a resistance of filament is small and this constant-current is proportional to the output voltage of the inverter.
On the other hand, a metallic vapor deposited film (mainly composed of tungsten with electric resistance of several hundred Q) is produced between stems of a discharge lamp by spatter of filament and when the filament is burnt out, constant-current supplied from the capacitor flows to the metallic vapor deposited film and consumes electric power. Therefore, if the tube diameter is too small, flares and stems of a thin discharge lamp are dissolved and the dissolved flares and stems contact the tube wall and may crack the tube. Further, when resin member is used for component parts of the discharge lamp, for example, electrode sockets, there is the danger that this electrode sockets are dissolved or ignited. It was therefore difficult to make the tube diameter of discharge lamps too small.
On a conventional discharge lamp lighting apparatus with capacitors connected in parallel to the discharge lamps for preheating filaments described above, constant-current supplied from the capacitors flows to a metallic vapor deposited film formed by the spatter consumes electric power when a filament is burned out. Because of this, if a tube diameter is made too small, flares and stems of discharge lamps are dissolved and the lamp crack is caused when dissolved flares and stems contact the tube wall. Further, when resin members are used for component parts, they can be dissolved or ignited. It was therefore difficult to make the tube diameter of discharge lamps too small.